Photomicrograph made with a Scanning Electron Microscope (SEM): Fly ash particles at 2,000× magnification. Most of the particles in this aerosol are nearly spherical.

An aerosol is defined as a suspension system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air.[1]Frederick G. Donnan presumably first used the term aerosol during World War I to describe an aero-solution, clouds of microscopic particles in air. This term developed analogously to the term hydrosol, a colloid system with water as the dispersed medium.[4]Primary aerosols contain particles introduced directly into the gas; secondary aerosols form through gas-to-particle conversion.[5]

Various types of aerosol, classified according to physical form and how they were generated, include dust, fume, mist, smoke and fog.[6]

There are several measures of aerosol concentration. Environmental science and health often uses the mass concentration (M), defined as the mass of particulate matter per unit volume with units such as μg/m3. Also commonly used is the number concentration (N), the number of particles per unit volume with units such as number/m3 or number/cm3.[7]

The size of particles has a major influence on their properties, and the aerosol particle radius or diameter (dp) is a key property used to characterise aerosols.

Aerosols vary in their dispersity. A monodisperse aerosol, producible in the laboratory, contains particles of uniform size. Most aerosols, however, as polydisperse colloidal systems, exhibit a range of particle sizes.[8] Liquid droplets are almost always nearly spherical, but scientists use an equivalent diameter to characterize the properities of various shapes of solid particles, some very irregular. The equivalent diameter is the diameter of a spherical particle with the same value of some physical property as the irregular particle.[9] The equivalent volume diameter (de) is defined as the diameter of a sphere of the same volume as that of the irregular particle.[10] Also commonly used is the aerodynamic diameter.

The same hypothetical log-normal aerosol distribution plotted, from top to bottom, as a number vs. diameter distribution, a surface area vs. diameter distribution, and a volume vs. diameter distribution. Typical mode names are shows at the top. Each distribution is normalized so that the total area is 1000.

For a monodisperse aerosol, a single number—the particle diameter—suffices to describe the size of the particles. However, more complicated particle-size distributions describe the sizes of the particles in a polydisperse aerosol. This distribution defines the relative amounts of particles, sorted according to size.[11] One approach to defining the particle size distribution uses a list of the sizes of every particle in a sample. However, this approach proves tedious to ascertain in aerosols with millions of particles and awkward to use. Another approach splits the complete size range into intervals and finds the number (or proportion) of particles in each interval. One then can visualize these data in a histogram with the area of each bar representing the proportion of particles in that size bin, usually normalised by dividing the number of particles in a bin by the width of the interval so that the area of each bar is proportionate to the number of particles in the size range that it represents.[12] If the width of the bins tends to zero, one gets the frequency function:[13]

One also usefully can approximate the particle size distribution using a mathematical function. The normal distribution usually does not suitably describe particle size distributions in aerosols because of the skewness associated a long tail of larger particles. Also for a quantity that varies over a large range, as many aerosol sizes do, the width of the distribution implies negative particles sizes, clearly not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain pollen grains and spores.[16]

For low values of the Reynolds number (<1), true for most aerosol motion, Stokes' law describes the force of resistance on a solid spherical particle in a fluid. However, Stokes' law is only valid when the velocity of the gas at the surface of the particle is zero. For small particles (< 1 μm) that characterize aerosols, however, this assumption fails. To account for this failure, one can introduce the Cunningham correction factor, always greater than 1. Including this factor, one finds the relation between the resisting force on a particle and its velocity:[19]

FD=3πηVdCc{\displaystyle F_{D}={\frac {3\pi \eta Vd}{C_{c}}}}

where

FD{\displaystyle F_{D}} is the resisting force on a spherical particle

VTS{\displaystyle V_{TS}} is the terminal settling velocity of the particle.

The terminal velocity can also be derived for other kinds of forces. If Stokes' law holds, then the resistance to motion is directly proportional to speed. The constant of proportionality is the mechanical mobility (B) of a particle:[21]

To account for the effect of the shape of non-spherical particles, a correction factor known as the dynamic shape factor is applied to Stokes' law. It is defined as the ratio of the resistive force of the irregular particle to that of a spherical particle with the same volume and velocity:[23]

One can apply the aerodynamic diameter to particulate pollutants or to inhaled drugs to predict where in the respiratory tract such particles deposit. Pharmaceutical companies typically use aerodynamic diameter, not geometric diameter, to characterize particles in inhalable drugs.[citation needed]

The previous discussion focussed on single aerosol particles. In contrast, aerosol dynamics explains the evolution of complete aerosol populations. The concentrations of particles will change over time as a result of many processes. External processes that move particles outside a volume of gas under study include diffusion, gravitational settling, and electric charges and other external forces that cause particle migration. A second set of processes internal to a given volume of gas include particle formation (nucleation), evaporation, chemical reaction, and coagulation.[26]

A differential equation called the Aerosol General Dynamic Equation (GDE) characterizes the evolution of the number density of particles in an aerosol due to these processes.[26]

As particles and droplets in an aerosol collide with one another, they may undergo coalescence or aggregation. This process leads to a change in the aerosol particle-size distribution, with the mode increasing in diameter as total number of particles decreases.[27] On occasion, particles may shatter apart into numerous smaller particles; however, this process usually occurs primarily in particles too large for consideration as aerosols.

The Knudsen number of the particle define three different dynamical regimes that govern the behaviour of an aerosol:

Kn=2λd{\displaystyle K_{n}={\frac {2\lambda }{d}}}

where λ{\displaystyle \lambda } is the mean free path of the suspending gas and d{\displaystyle d} is the diameter of the particle.[28] For particles in the free molecular regime, Kn >> 1; particles small compared to the mean free path of the suspending gas.[29] In this regime, particles interact with the suspending gas through a series of "ballistic" collisions with gas molecules. As such, they behave similarly to gas molecules, tending to follow streamlines and diffusing rapidly through Brownian motion. The mass flux equation in the free molecular regime is:

where a is the particle radius, P∞ and PA are the pressures far from the droplet and at the surface of the droplet respectively, kb is the Boltzmann constant, T is the temperature, CA is mean thermal velocity and α is mass accommodation coefficient.[citation needed] The derivation of this equation assumes constant pressure and constant diffusion coefficient.

Particles are in the continuum regime when Kn << 1.[29] In this regime, the particles are big compared to the mean free path of the suspending gas, meaning that the suspending gas acts as a continuous fluid flowing round the particle.[29] The molecular flux in this regime is:

where a is the radius of the particle A, MA is the molecular mass of the particle A, DAB is the diffusion coefficient between particles A and B, R is the ideal gas constant, T is the temperature (in absolute units like kelvin), and PA∞ and PAS are the pressures at infinite and at the surface respectively.[citation needed]

The transition regime contains all the particles in between the free molecular and continuum regimes or Kn ≈ 1. The forces experienced by a particle are a complex combination of interactions with individual gas molecules and macroscopic interactions. The semi-empirical equation describing mass flux is:

Aerosol partitioning theory governs condensation on and evaporation from an aerosol surface, respectively. Condensation of mass causes the mode of the particle-size distributions of the aerosol to increase; conversely, evaporation causes the mode to decrease. Nucleation is the process of forming aerosol mass from the condensation of a gaseous precursor, specifically a vapour. Net condensation of the vapour requires supersaturation, a partial pressure greater than its vapour pressure. This can happen for three reasons:[citation needed]

Lowering the temperature of the system lowers the vapour pressure.

Chemical reactions may increase the partial pressure of a gas or lower its vapour pressure.

The addition of additional vapour to the system may lower the equilibrium vapour pressure according to Raoult's law.

There are two types of nucleation processes. Gases preferentially condense onto surfaces of pre-existing aerosol particles, known as heterogeneous nucleation. This process causes the diameter at the mode of particle-size distribution to increase with constant number concentration.[30] With sufficiently high supersaturation and no suitable surfaces, particles may condense in the absence of a pre-existing surface, known as homogeneous nucleation. This results in the addition of very small, rapidly growing particles to the particle-size distribution.[30]

Water coats particles in an aerosols, making them activated, usually in the context of forming a cloud droplet.[citation needed] Following the Kelvin equation (based on the curvature of liquid droplets), smaller particles need a higher ambient relative humidity to maintain equilibrium than larger particles do. The following formula gives relative humidity at equilibrium:

where ps{\displaystyle p_{s}} is the saturation vapor pressure above a particle at equilibrium (around a curved liquid droplet), p0 is the saturation vapor pressure (flat surface of the same liquid) and S is the saturation ratio.

Stability of nanoparticle agglomerates is critical for estimating size distribution of aerosolized particles from nano-powders or other sources. At nanotechnology workplaces, workers can be exposed via inhalation to potentially toxic substances during handling and processing of nanomaterials. Nanoparticles in the air often form agglomerates due to attractive inter-particle forces, such as van der Waals force or electrostatic force if the particles are charged. As a result, aerosol particles are usually observed as agglomerates rather than individual particles. For exposure and risk assessments of airborne nanoparticles, it is important to know about the size distribution of aerosols. When inhaled by humans, particles with different diameters are deposited in varied locations of the central and periphery respiratory system. Particles in nanoscale have been shown to penetrate the air-blood barrier in lungs and be translocated into secondary organs in the human body, such as the brain, heart and liver. Therefore, the knowledge on stability of nanoparticle agglomerates is important for predicting the size of aerosol particles, which helps assess the potential risk of them to human bodies.

Different experimental systems have been established to test the stability of airborne particles and their potentials to deagglomerate under various conditions. A comprehensive system recently reported by Ding & Riediker (2015)[44] is able to maintain robust aerosolization process and generate aerosols with stable number concentration and mean size from nano-powders. The deagglomeration potential of various airborne nanomaterials can be also studied using critical orifices. This process was also investigated by Stahlmecke et al. (2009).[45] In addition, an impact fragmentation device was developed to investigate bonding energies between particles.[46]

A standard deagglomeration testing procedure could be foreseen with the developments of the different types of existing systems. The likeliness of deagglomeration of aerosol particles in occupational settings can be possibly ranked for different nanomaterials if a reference method is available. For this purpose, inter-laboratory comparison of testing results from different setups could be launched in order to explore the influences of system characteristics on properties of generated nanomaterials aerosols.

Particles can deposit in the nose, mouth, pharynx and larynx (the head airways region), deeper within the respiratory tract (from the trachea to the terminal bronchioles), or in the alveolar region.[47] The location of deposition of aerosol particles within the respiratory system strongly determines the health effects of exposure to such aerosols.[48] This phenomenon led people to invent aerosol samplers that select a subset of the aerosol particles that reach certain parts of the respiratory system.[49] Examples of these subsets of the particle-size distribution of an aerosol, important in occupational health, include the inhalable, thoracic, and respirable fractions. The fraction that can enter each part of the respiratory system depends on the deposition of particles in the upper parts of the airway.[50] The inhalable fraction of particles, defined as the proportion of particles originally in the air that can enter the nose or mouth, depends on external wind speed and direction and on the particle-size distribution by aerodynamic diameter.[51] The thoracic fraction is the proportion of the particles in ambient aerosol that can reach the thorax or chest region.[52] The respirable fraction is the proportion of particles in the air that can reach the alveolar region.[53] To measure the respirable fraction of particles in air, a pre-collector is used with a sampling filter. The pre-collector excludes particles as the airways remove particles from inhaled air. The sampling filter collects the particles for measurement. It is common to use cyclonic separation for the pre-collector, but other techniques include impactors, horizontal elutriators, and large pore membrane filters.[54]

Two alternative size-selective criteria, often used in atmospheric monitoring, are PM10 and PM2.5. PM10 is defined by ISO as particles which pass through a size-selective inlet with a 50% efficiency cut-off at 10 μm aerodynamic diameter and PM2.5 as particles which pass through a size-selective inlet with a 50% efficiency cut-off at 2.5 μm aerodynamic diameter. PM10 corresponds to the “thoracic convention” as defined in ISO 7708:1995, Clause 6; PM2.5 corresponds to the “high-risk respirable convention” as defined in ISO 7708:1995, 7.1.[55] The United States Environmental Protection Agency replaced the older standards for particulate matter based on Total Suspended Particulate with another standard based on PM10 in 1987[56] and then introduced standards for PM2.5 (also known as fine particulate matter) in 1997.[57]

Several types of atmospheric aerosol have a significant effect on Earth's climate: volcanic, desert dust, sea-salt, that originating from biogenic sources and human-made. Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years, and reflect sunlight, lowering temperature. Desert dust, mineral particles blown to high altitudes, absorb heat and may be responsible for inhibiting storm cloud formation. Human-made sulfate aerosols, primarily from burning oil and coal, affect the behavior of clouds.[58]

Although all hydrometeors, solid and liquid, can be described as aerosols, a distinction is commonly made between such dispersions (i.e. clouds) containing activated drops and crystals, and aerosol particles. The atmosphere of Earth contains aerosols of various types and concentrations, including quantities of:

Aerosols interact with the Earth's energy budget in two ways, directly and indirectly.

E.g., a direct effect is that aerosols scatter and absorb incoming solar radiation. This will mainly lead to a cooling of the surface (solar radiation is scattered back to space) but may also contribute to a warming of the surface (caused by the absorption of incoming solar energy).[60] This will be an additional element to the greenhouse effect and therefore contributing to the global climate change.[61]

The indirect effects refer to the aerosols interfering with formations that interact directly with radiation. For example, they are able to modify the size of the cloud particles in the lower atmosphere, thereby changing the way clouds reflect and absorb light and therefore modifying the Earth's energy budget.[59]

When aerosols absorb pollutants, it facilitates the deposition of pollutants to the surface of the earth as well as to bodies of water.[61] This has the potential to be damaging to both the environment and human health.

Aerosol particles with an effective diameter smaller than 10 μm can enter the bronchi, while the ones with an effective diameter smaller than 2.5 μm can enter as far as the gas exchange region in the lungs,[62] which can be hazardous to human health.

1.
Solid
–
Solid is one of the four fundamental states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume, unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to other, either in a regular geometric lattice or irregularly. The branch of physics deals with solids is called solid-state physics. Materials science is concerned with the physical and chemical properties of solids. Solid-state chemistry is concerned with the synthesis of novel materials, as well as the science of identification. The atoms, molecules or ions which make up solids may be arranged in a repeating pattern. Materials whose constituents are arranged in a regular pattern are known as crystals, in some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a single crystal. Almost all common metals, and many ceramics, are polycrystalline, in other materials, there is no long-range order in the position of the atoms. These solids are known as amorphous solids, examples include polystyrene, whether a solid is crystalline or amorphous depends on the material involved, and the conditions in which it was formed. Solids which are formed by slow cooling will tend to be crystalline, likewise, the specific crystal structure adopted by a crystalline solid depends on the material involved and on how it was formed. While many common objects, such as an ice cube or a coin, are chemically identical throughout, for example, a typical rock is an aggregate of several different minerals and mineraloids, with no specific chemical composition. Wood is an organic material consisting primarily of cellulose fibers embedded in a matrix of organic lignin. In materials science, composites of more than one constituent material can be designed to have desired properties, the forces between the atoms in a solid can take a variety of forms. For example, a crystal of sodium chloride is made up of sodium and chlorine. In diamond or silicon, the atoms share electrons and form covalent bonds, in metals, electrons are shared in metallic bonding. Some solids, particularly most organic compounds, are together with van der Waals forces resulting from the polarization of the electronic charge cloud on each molecule. The dissimilarities between the types of solid result from the differences between their bonding, metals typically are strong, dense, and good conductors of both electricity and heat

2.
Drop (liquid)
–
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the end of a tube or other surface boundary. Drops may also be formed by the condensation of a vapor or by atomization of a mass of liquid. Liquid forms drops because the liquid surface tension. A simple way to form a drop is to allow liquid to flow slowly from the end of a vertical tube of small diameter. The surface tension of the causes the liquid to hang from the tube. When the drop exceeds a size it is no longer stable. The falling liquid is also a drop held together by surface tension, in the pendant drop test, a drop of liquid is suspended from the end of a tube by surface tension. The force due to surface tension is proportional to the length of the boundary between the liquid and the tube, with the proportionality constant usually denoted γ. Since the length of boundary is the circumference of the tube. The limit of this formula, as α goes to 90°, gives the weight of a pendant drop for a liquid with a given surface tension. M g = π d γ This relationship is the basis of a convenient method of measuring surface tension, more sophisticated methods are available to take account of the developing shape of the pendant as the drop grows. These methods are used if the tension is unknown. In medicine, droppers have a diameter, in such a way that 1 millilitre is equivalent to 20 drops. And, for the cases when smaller amounts are necessary, microdroppers are used, the drop adhesion to a solid can be divided to two categories, lateral adhesion and normal adhesion. Normal adhesion is the required to detach a drop from the surface in the normal direction. The measurement of both forms can be done with the Centrifugal Adhesion Balance. The CAB uses a combination of centrifugal and gravitational forces to any ratio of lateral and normal forces

3.
Atmosphere of Earth
–
The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earths gravity. The atmosphere of Earth protects life on Earth by absorbing solar radiation, warming the surface through heat retention. By volume, dry air contains 78. 09% nitrogen,20. 95% oxygen,0. 93% argon,0. 04% carbon dioxide, and small amounts of other gases. Air also contains an amount of water vapor, on average around 1% at sea level. The atmosphere has a mass of about 5. 15×1018 kg, the atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km, or 1. 57% of Earths radius, is used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km, several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition. The study of Earths atmosphere and its processes is called atmospheric science, early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The three major constituents of air, and therefore of Earths atmosphere, are nitrogen, oxygen, water vapor accounts for roughly 0. 25% of the atmosphere by mass. The remaining gases are often referred to as gases, among which are the greenhouse gases, principally carbon dioxide, methane, nitrous oxide. Filtered air includes trace amounts of other chemical compounds. Various industrial pollutants also may be present as gases or aerosols, such as chlorine, fluorine compounds, sulfur compounds such as hydrogen sulfide and sulfur dioxide may be derived from natural sources or from industrial air pollution. In general, air pressure and density decrease with altitude in the atmosphere, however, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions. In this way, Earths atmosphere can be divided into five main layers, excluding the exosphere, Earth has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere. It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km where it merges into the solar wind. This layer is composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another, thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind, the exosphere is located too far above Earth for any meteorological phenomena to be possible

4.
Gas
–
Gas is one of the four fundamental states of matter. A pure gas may be made up of atoms, elemental molecules made from one type of atom. A gas mixture would contain a variety of pure gases much like the air, what distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer, the interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image. One type of commonly known gas is steam, the gaseous state of matter is found between the liquid and plasma states, the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention, high-density atomic gases super cooled to incredibly low temperatures are classified by their statistical behavior as either a Bose gas or a Fermi gas. For a comprehensive listing of these states of matter see list of states of matter. The only chemical elements which are stable multi atom homonuclear molecules at temperature and pressure, are hydrogen, nitrogen and oxygen. These gases, when grouped together with the noble gases. Alternatively they are known as molecular gases to distinguish them from molecules that are also chemical compounds. The word gas is a neologism first used by the early 17th-century Flemish chemist J. B. van Helmont, according to Paracelsuss terminology, chaos meant something like ultra-rarefied water. An alternative story is that Van Helmonts word is corrupted from gahst and these four characteristics were repeatedly observed by scientists such as Robert Boyle, Jacques Charles, John Dalton, Joseph Gay-Lussac and Amedeo Avogadro for a variety of gases in various settings. Their detailed studies ultimately led to a relationship among these properties expressed by the ideal gas law. Gas particles are separated from one another, and consequently have weaker intermolecular bonds than liquids or solids. These intermolecular forces result from interactions between gas particles. Like-charged areas of different gas particles repel, while oppositely charged regions of different gas particles attract one another, transient, randomly induced charges exist across non-polar covalent bonds of molecules and electrostatic interactions caused by them are referred to as Van der Waals forces. The interaction of these forces varies within a substance which determines many of the physical properties unique to each gas. A comparison of boiling points for compounds formed by ionic and covalent bonds leads us to this conclusion, the drifting smoke particles in the image provides some insight into low pressure gas behavior

5.
Human impact on the environment
–
Human impact on the environment or anthropogenic impact on the environment includes impacts on biophysical environments, biodiversity, and other resources. The term anthropogenic designates an effect or object resulting from human activity, the atmospheric scientist Paul Crutzen introduced the term Anthropocene in the mid-1970s. The term is used in the context of pollution emissions that are produced as a result of human activities but applies broadly to all major human impacts on the environment. Environmental impacts caused by the application of technology are often perceived as unavoidable for several reasons, third, according to the second law of thermodynamics, order can be increased within a system only by increasing disorder or entropy outside the system. Thus, technologies can create “order” in the economy only at the expense of increasing “disorder” in the environment. According to a number of studies, increased entropy is likely to be correlated to environmental impacts. The environmental impact of agriculture based on the wide variety of agricultural practices employed around the world. Ultimately, the impact depends on the production practices of the system used by farmers. The connection between emissions into the environment and the system is indirect, as it also depends on other climate variables such as rainfall. An example of a means-based indicator would be the quality of groundwater, an indicator reflecting the loss of nitrate to groundwater would be effect-based. The environmental impact of agriculture involves a variety of factors from the soil, to water, the air, animal and soil diversity, plants, and the food itself. Some of the issues that are related to agriculture are climate change, deforestation, genetic engineering, irrigation problems, pollutants, soil degradation. These conservation issues are part of conservation, and are addressed in fisheries science programs. There is a gap between how many fish are available to be caught and humanity’s desire to catch them, a problem that gets worse as the world population grows. The journal Science published a study in November 2006, which predicted that, at prevailing trends. Many countries, such as Tonga, the United States, Australia and New Zealand, the impacts stem from the changed hydrological conditions owing to the installation and operation of the scheme. An irrigation scheme draws water from the river and distributes it over the irrigated area. These may be called direct effects, effects on soil and water quality are indirect and complex, and subsequent impacts on natural, ecological and socio-economic conditions are intricate

6.
Fog
–
Fog consists of visible cloud water droplets or ice crystals suspended in the air at or near the Earths surface. Fog can be considered a type of low-lying cloud and is influenced by nearby bodies of water, topography. In turn, fog has affected many human activities, such as shipping, travel, the term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally. By definition, fog reduces visibility to less than 1 kilometre, for aviation purposes in the UK, a visibility of less than 5 kilometres but greater than 999 metres is considered to be mist if the relative humidity is 70% or greater, below 70%, haze is reported. Fog forms when the difference between air temperature and dew point is less than 2.5 °C or 4 °F, Fog begins to form when water vapor condenses into tiny liquid water droplets suspended in the air. Water vapor normally begins to condense on condensation nuclei such as dust, ice, Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. Fog normally occurs at a relative humidity near 100% and this occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidities, and can fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus, Fog can form suddenly and can dissipate just as rapidly. The sudden formation of fog is known as flash fog, Fog commonly produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity of fog attains 100% and the cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point. The inversion boundary varies its altitude primarily in response to the weight of the air above it, the marine layer, and any fogbank it may contain, will be squashed when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering. Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred, radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The warm ground produces condensation in the air by heat conduction. In perfect calm the fog layer can be less than a meter deep, radiation fogs occur at night, and usually do not last long after sunrise, but they can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter, examples of this phenomenon include the Tule fog. Ground fog is fog that obscures less than 60% of the sky, advection fog occurs when moist air passes over a cool surface by advection and is cooled

7.
Geyser
–
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditions that exist in only a few places on Earth, generally all geyser field sites are located near active volcanic areas, and the geyser effect is due to the proximity of magma. Generally, surface water works its way down to a depth of around 2,000 metres where it contacts hot rocks. The resultant boiling of the water results in the geyser effect of hot water. Over one thousand known geysers exist worldwide, at least 1,283 geysers have erupted in Yellowstone National Park, Wyoming, United States, and an average of 465 geysers are active there in a given year. Like many other phenomena, geysers are not unique to planet Earth. Jet-like eruptions, often referred to as cryogeysers, have been observed on several of the moons of the solar system. Due to the low ambient pressures, these eruptions consist of vapor without liquid, they are more easily visible by particles of dust. Water vapor jets have been observed near the pole of Saturns moon Enceladus. There are also signs of carbon dioxide eruptions from the polar ice cap of Mars. In the latter two cases, instead of being driven by energy, the eruptions seem to rely on solar heating via a solid-state greenhouse effect. The word geyser comes from Geysir, the name of a spring at Haukadalur, Iceland, that name, in turn, comes from the Icelandic verb geysa, to gush. Geysers are generally associated with volcanic areas, as the water boils, the resulting pressure forces a superheated column of steam and water to the surface through the geysers internal plumbing. The formation of geysers specifically requires the combination of three conditions that are usually found in volcanic terrain. The heat needed for geyser formation comes from magma that needs to be near the surface of the earth, the fact that geysers need heat much higher than normally found near the earths surface is the reason they are associated with volcanoes or volcanic areas. The pressures encountered at the areas where the water is heated make the point of the water much higher than at normal atmospheric pressures. The water ejected from a geyser travels underground through deep, pressurized fissures in the Earths crust, in order for the heated water to form a geyser, a plumbing system made of fractures, fissures, porous spaces, and sometimes cavities is required. This includes a reservoir to hold the water while it is being heated, Geysers are generally aligned along faults

8.
Haze
–
Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. Sources for haze particles include farming, traffic, industry, seen from afar and depending upon the direction of view with respect to the sun, haze may appear brownish or bluish, while mist tends to be bluish-grey. Whereas haze often is thought of as a phenomenon of dry air, however, haze particles may act as condensation nuclei for the subsequent formation of mist droplets, such forms of haze are known as wet haze. The term haze, in literature, generally is used to denote visibility-reducing aerosols of the wet type. Such aerosols commonly arise from complex chemical reactions occur as sulfur dioxide gases emitted during combustion are converted into small droplets of sulphuric acid. The reactions are enhanced in the presence of sunlight, high relative humidity, a small component of wet haze aerosols appear to be derived from compounds released by trees, such as terpenes. For all these reasons, wet haze tends to be primarily a warm-season phenomenon, large areas of haze covering many thousands of kilometers may be produced under favorable conditions each summer. Haze often occurs when dust and smoke particles accumulate in relatively dry air, when weather conditions block the dispersal of smoke and other pollutants they concentrate and form a usually low-hanging shroud that impairs visibility and may become a respiratory health threat. Industrial pollution can result in dense haze, which is known as smog, since 1991, haze has been a particularly acute problem in Southeast Asia. The main source of the haze has been occurring in Sumatra. In response to the 1997 Southeast Asian haze, the ASEAN countries agreed on a Regional Haze Action Plan, in 2002, all ASEAN countries except Indonesia signed the Agreement on Transboundary Haze Pollution, but the pollution is still a problem today. Under the agreement the ASEAN secretariat hosts a co-ordination and support unit, during the 2013 Southeast Asian haze, Singapore experienced a record high pollution level, with the 3-hour Pollution Standards Index reaching a record high of 401. A full list of areas is available on EPAs website. Haze is no longer a domestic problem and it has become one of the causes of international disputes among neighboring countries. Haze migrates to adjacent countries and thereby pollutes other countries as well, one of the most recent problems concerned the two neighboring countries Malaysia and Indonesia. Winds blow most of the fumes across the narrow Strait of Malacca to Malaysia, the 2015 Southeast Asian haze constitutes an ongoing crisis. Haze causes issues in the area of photography, where the penetration of large amounts of dense atmosphere may be necessary to image distant subjects. This results in the effect of a loss of contrast in the subject

9.
Air pollution
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Air pollution occurs when harmful substances including particulates and biological molecules are introduced into Earths atmosphere. It may cause diseases, allergies or death in humans, it may cause harm to other living organisms such as animals and food crops. Human activity and natural processes can both generate air pollution, indoor air pollution and poor urban air quality are listed as two of the worlds worst toxic pollution problems in the 2008 Blacksmith Institute Worlds Worst Polluted Places report. According to the 2014 WHO report, air pollution in 2012 caused the deaths of around 7 million people worldwide, an air pollutant is a substance in the air that can have adverse effects on humans and the ecosystem. The substance can be particles, liquid droplets, or gases. A pollutant can be of natural origin or man-made, pollutants are classified as primary or secondary. Primary pollutants are usually produced from a process, such as ash from a volcanic eruption, other examples include carbon monoxide gas from motor vehicle exhaust, or the sulfur dioxide released from factories. Secondary pollutants are not emitted directly, rather, they form in the air when primary pollutants react or interact. Ground level ozone is a prominent example of a secondary pollutant, some pollutants may be both primary and secondary, they are both emitted directly and formed from other primary pollutants. Substances emitted into the atmosphere by human activity include, Carbon dioxide - Debate continues over whether carbon dioxide should be classified as an atmospheric pollutant, because of its role as a greenhouse gas it has been described as the leading pollutant and the worst climate pollution. Against this it is argued that carbon dioxide is a component of the atmosphere, essential for plant life. This question of terminology has practical effects, for example as determining whether the U. S. Clean Air Act is deemed to regulate CO2 emissions, CO2 increase in earths atmosphere has been accelerating. Sulfur oxides - particularly sulfur dioxide, a compound with the formula SO2. SO2 is produced by volcanoes and in industrial processes. Coal and petroleum often contain sulfur compounds, and their combustion generates sulfur dioxide, further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. This is one of the causes for concern over the impact of the use of these fuels as power sources. Nitrogen oxides - Nitrogen oxides, particularly nitrogen dioxide, are expelled from high temperature combustion and they can be seen as a brown haze dome above or a plume downwind of cities. Nitrogen dioxide is a compound with the formula NO2

10.
Smoke
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It is commonly an unwanted by-product of fires, but may also be used for pest control, communication, defensive and offensive capabilities in the military, cooking, or smoking. Smoke is used in rituals where incense, sage, or resin is burned to produce a smell for spiritual purposes, smoke is sometimes used as a flavoring agent, and preservative for various foodstuffs. Smoke is also a component of internal combustion engine exhaust gas, smoke inhalation is the primary cause of death in victims of indoor fires. The smoke kills by a combination of damage, poisoning and pulmonary irritation caused by carbon monoxide, hydrogen cyanide. Smoke is an aerosol of solid particles and liquid droplets that are close to the range of sizes for Mie scattering of visible light. This effect has been likened to three-dimensional textured privacy glass — a smoke cloud does not obstruct an image, the composition of smoke depends on the nature of the burning fuel and the conditions of combustion. High temperature also leads to production of nitrogen oxides, sulfur content yields sulfur dioxide, or in case of incomplete combustion, hydrogen sulfide. Carbon and hydrogen are almost completely oxidized to carbon dioxide and water, fires burning with lack of oxygen produce a significantly wider palette of compounds, many of them toxic. Partial oxidation of carbon produces carbon monoxide, nitrogen-containing materials can yield hydrogen cyanide, ammonia, hydrogen gas can be produced instead of water. Content of halogens such as chlorine may lead to production of e. g. hydrogen chloride, phosgene, dioxin, hydrogen fluoride can be formed from fluorocarbons, whether fluoropolymers subjected to fire or halocarbon fire suppression agents. Phosphorus and antimony oxides and their products can be formed from some fire retardant additives. Heterocyclic compounds may be also present, heavier hydrocarbons may condense as tar, smoke with significant tar content is yellow to brown. Partial oxidation of the released hydrocarbons yields in a palette of other compounds, aldehydes, ketones, alcohols. The visible particulate matter in such smokes is most commonly composed of carbon, other particulates may be composed of drops of condensed tar, or solid particles of ash. The presence of metals in the fuel yields particles of metal oxides, particles of inorganic salts may also be formed, e. g. ammonium sulfate, ammonium nitrate, or sodium chloride. Inorganic salts present on the surface of the particles may make them hydrophilic. Many organic compounds, typically the aromatic hydrocarbons, may be adsorbed on the surface of the solid particles. Metal oxides can be present when metal-containing fuels are burned, e. g. solid rocket fuels containing aluminium, depleted uranium projectiles after impacting the target ignite, producing particles of uranium oxides

The same hypothetical log-normal aerosol distribution plotted, from top to bottom, as a number vs. diameter distribution, a surface area vs. diameter distribution, and a volume vs. diameter distribution. Typical mode names are shows at the top. Each distribution is normalized so that the total area is 1000.

In probability theory, the normal (or Gaussian) distribution is a very common continuous probability distribution. …

The bean machine, a device invented by Francis Galton, can be called the first generator of normal random variables. This machine consists of a vertical board with interleaved rows of pins. Small balls are dropped from the top and then bounce randomly left or right as they hit the pins. The balls are collected into bins at the bottom and settle down into a pattern resembling the Gaussian curve.

For the normal distribution, the values less than one standard deviation away from the mean account for 68.27% of the set; while two standard deviations from the mean account for 95.45%; and three standard deviations account for 99.73%.

A typical paint valve system will have a "female" valve, the stem being part of the top actuator. The valve can be preassembled with the valve cup and installed on the can as one piece, prior to pressure-filling. The actuator is added later.

A diffusion is a process in physics. Some particles are dissolved in a glass of water. At first, the particles are all near one corner of the glass. If the particles randomly move around ("diffuse") in the water, they eventually become distributed randomly and uniformly from an area of high concentration to an area of low concentration, and organized (diffusion continues, but with no net flux).